In this final part of our Rheology series we conclude with a discussion of the rheological properties of lubricating grease. Two key characteristics of grease are consistency and viscosity. It is therefore not surprising that these rheological properties appear in all grease specifications and product literature. Another key rheological characteristic of grease that is often overlooked, is deformation (visco-elasticity). The deformation behaviour of lubricating grease under shear or stress is critical in the satisfactory performance of the grease.
The smooth and reliable operation of oil lubricated machinery depends on consistent lubrication between moving surfaces. To achieve this the lubricating oil must be of Newtonian nature to uphold a robust film to separate the moving surfaces. The viscosity of Newtonian oils does not change with shear, stress or rate of flow.
For most machines in intermittent service it is desirable to have a non-Newtonian lubricant with a thicker consistency to remain in place during shutdown, but it must, however, have the fluidity of an oil when the equipment is in operation.
The American Society for Testing and Materials (ASTM) defines lubricating grease as “A solid to semifluid product of dispersion of a thickening agent in liquid lubricant. Other ingredients imparting special properties may be included” The thickener holds the oil in place but releases it under shear or stress to give the effect of oil lubrication with similar viscosity as the base oil used in the grease. When the shear/stress is removed, the thickener ‘absorbs’ the oil again and the grease thickens to its original consistency. In this instance the rheological behaviour of the non-Newtonian grease is thixotropic. In fact, most greases available today are of thixotropic nature.
In some instances a non-Newtonian grease that hardens when subjected to stress is preferred. Typical examples are large opencast mining machines where the grease must be pumped long distances from a central grease reservoir to the actual lubrication points. The grease needs to be soft enough to be pumped and then thickens under mechanical pressure to adhere to heavily loaded components and provide superior lubrication, especially under shockload conditions. The rheological behaviour of such non-Newtonian greases is rheopectic. These greases require a gentle touch when being pumped. It is crucial to employ a special pumping system with low shear for rheopectic greases. High shear forces can accelerate the thickening process and may cause grease feed lines to get clogged. Tailor-made pumps are therefore required that deliver a smooth, controlled flow to preserve the rheopectic nature of the grease.
Q8Oils offer a comprehensive range of high-quality greases for a wide variety of automotive, commercial, construction, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.